An electronic control unit connected to a communication line, the electronic control unit including: a transmission request buffer including a plurality of transmission buffer areas; a message buffer including a plurality of message buffer areas; a transmission controller configured to transmit data stored in the message buffer to the communication line; and a central processing unit. The central processing unit programmed to search for data with a higher priority than data stored in the message buffer from among pieces of data stored in the transmission request buffer, sequentially search for a vacant buffer area in which no data is stored from among the plurality of message buffer areas, set the data that has been searched for in the vacant buffer area that has been searched for.